Process for distilling water and distillation apparatus

ABSTRACT

A process and an apparatus for distilling water in which a heat receiving plate member and a plurality of cooling plate members have a means for holding water to be treated on each reverse side thereof and are arranged in parallel spaced relationship with each other, and the water vapor produced by heating the heat receiving surface of the heat receiving plate member is condensed on the condensing surface of the adjacent cooling plate member, while giving the latent heat of the condensation to the cooling plate member and heating a water held on its back to produce the water vapor which is condensed on the condensing surface of the next cooling plate member, such evaporation by the latent heat and condensation being conducted according the number of rooms partitioned by adjacent two cooling plate members. There are apparatuses of two types, i.e. thermal diffusion type and convection type, and they are availably employed for purifying impure water or desalting sea water in high heat efficiency by utilizing various heat sources such as solar heat.

BACKGROUND OF THE INVENTION

The present invention relates to a process for distilling water such assea water and a distillation apparatus.

Various distillation processes and apparatuses are known such as amultiple effect evaporator, a multistage flash evaporator and a solardistillation apparatus, and are availably employed for purifying impurewater or desalting sea water.

A known distillation apparatus such as a multistage flash evaporator iscomplicated and large, and the heat efficiency is not so good. Theevaporator has also the disadvantages that it is necessary to reducepressure or to make vacuous, that a large mechanical energy is requiredfor conveying upward and downward water to be treated or for passing thewater through, for instance, spiral pipes, and that the maintenance costcomes expensive.

Solar distillation apparatuses hitherto used are classified roughly intoa so-called basin type in which a heat collector and an evaporator arejoined in a body and a so-called separate type in which a heat collectorand an evaporator are separately installed. The basin type solardistillation apparatus is generally inferior in the heat efficiency, butthe construction is simple. On the other hand, the separate type solardistillation apparatus is good in the heat efficiency, but theconstruction is complicated and the operation cost is expensive.

In general, a basin type solar distillation apparatus is composed of abasin filled with a waste water or sea water in the atmosphere and atransparent cover therefor. The basin is lined with a material such asbutyl rubber which is water resistant and absorbs well solar radiation.A heat insulating material is also employed for decreasing heat loss. Aglass sheet and a weather-proofing clear plastic film are employed asthe transparent cover, and the cover is installed with a slightgradient. The sunlight falls on the basin through the transparent coverand sea water so as to raise the temperature of sea water. When thetemperature of sea water becomes higher than the surrounding atmospherictemperature by about 10° to 15° C., the evaporation becomes vigorous andthe water vapor begins to circulate within the apparatus with air. Thewater vapor is condensed on the inner surface of the cover, and thewater droplets run along the inner surface of the inclined cover to theside walls of the basin and is collected into a fresh water tank througha conduit. In such basin type solar distillation apparatus, the heatefficiency is less than 45% in summer when the highest efficiency isattained, and is about 30% on the average throughout the year. In orderto improve the distillation efficiency, there have been proposed variousbasin type solar distillation methods and apparatuses, such as the useof a basin having a corrugated bottom to increase the evaporation areaand the use of a water or brine cooling means for cooling a coverinstead of air cooling, but they cannot raise the efficiency so high.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process fordistilling water in a high heat efficiency.

A further object of the invention is to provide an apparatus fordistilling water, the structure of which is very simple.

A still further object of the invention is to provide a multi effectdistillation apparatus having a high heat efficiency.

These and other objects of the invention will become apparent from thedescription hereinafter.

In accordance with the present invention, there is provided a processfor distilling water which comprises heating one side of a heatreceiving plate member, evaporating water to be treated which is held onthe other side of said heat receiving plate member, condensing theresulting water vapor on a condensing surface of a cooling plate memberwhich is the cooling plate member adjacent to said heat receiving platemember among a plurality of cooling plate members provided at a space toeach other and in parallel with said heat receiving plate member,repeating the evaporation of water to be treated which is held on theopposite side of each cooling plate member to the condensing surface andthe condensation of the resulting water vapor on the condensing surfaceof each cooling plate member, said evaporation being caused by thelatent heat of said condensation, and collecting the condensed water.

The present invention also provides an apparatus for distilling waterwhich comprises

(a) a casing,

(b) a heat receiving plate member for receiving heat on one side thereofand evaporating a water to be treated,

(c) a plurality of cooling plate members for condensing the water vaporand evaporating a water to be treated by the latent heat of thecondensation, said cooling plate members being arranged in parallelspaced relationship with said heat receiving plate member and with eachother,

(d) water holding means for holding said water to be treated on theother side of said heat receiving plate member and on one side of eachof said cooling plate members, said each water holding means beingopposite to the condensing surface of the adjacent cooling plate member,

(e) a water supply means for supplying the water to be treated to saidwater holding means, and

(f) a water collecting means for collecting the condensed water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic longitudinal section view of a solar distillationapparatus showing an embodiment of the present invention;

FIG. 2 is a partially cutway perspective view of the apparatus shown inFIG. 1;

FIG. 3 is a schematic longitudinal section view of a solar distillationapparatus showing a modification of the present invention;

FIG. 4 is a partially cutway perspective view of the apparatus shown inFIG. 3;

FIG. 5 is a partially cutway perspective view of a solar distillationapparatus showing another modification of the present invention;

FIG. 6 is a cross section view of a cyrindrical solar distillationapparatus showing still another modification of the present invention;

FIG. 7 is a schematic longitudinal section view of a convection typedistillation apparatus showing another embodiment of the presentinvention;

FIG. 8 is a partially cutway perspective view of a convection typedistillation apparatus showing the state of being disassembled; and

FIG. 9 is a longitudinal section view of a convection type solardistillation apparatus showing a modification of the present invention.

DETAILED DESCRIPTION

The present invention is applicable to purification of impure water suchas an industrial waste water, a domestic waste water, an undergroundwater or a liver water, and desalting of sea water.

In the present invention, heat is transferred by thermal diffusion orconvection, and the process of the present invention is practiced byemploying a multi effect distillation apparatus of a thermal diffusiontype or a convection type. As a heat source, there may be employedvarious heat sources such as the sunlight, electric heat, fuel, steam,waste heat and hot water.

One embodiment of the present invention is thermal diffusion type solardistillation process and apparatus. A part of a casing of the apparatusis made up of a transparent cover through which the sunlight transmitsand falls on a heat receiving plate member to cause it to heat and bywhich warmth inside the cover is also kept. A cooling plate member has areflective surface on one side thereof, and the water vapor condensesthereon. A porous water-absorptive sheet is employed as a water holdingmeans, and is attached to each of the back sides of the heat receivingplate member and the reflective cooling plate members with respect tothe heat receiving surface and the condensing surfaces. The transparentcover, the heat receiving plate member and a plurality of the coolingplate members are arranged in that order in parallel spaced relationshipwith each other so that the porous water-absorptive sheet faces to thecondensing surface of the cooling plate member. A water to be treated isabsorbed by the porous sheets. The water absorbed by the porous sheetattached to the back side of the heat receiving plate member is heatedby the heat receiving plate member to evaporate, and the evaporatedwater is condensed on the condensing surface of the cooling plate memberfacing the back side of the heat receiving plate member. The waterabsorbed by the porous sheet attached to this cooling plate member isheated and evaporated by the latent heat of the condensation and is thencondensed on the condensing surface of another cooling plate memberfacing the porous sheet stuck to the preceding cooling plate member. Inthe same manner, evaporation of water and condensation of the resultingwater vapor are repeated in order between the respective porous sheetsand condensing surfaces opposite to each other. The porous sheetattached to the last cooling plate member located outside is cooled bythe outside air, and also serves as a part of the casing.

By employing a heat receiving plate member having a high heat absorptionproperty and a reflective cooling plate member having a high reflectionproperty, the heat loss occurring at the porous sheet portions areprevented and a water to be treated is evaporated with a goodefficiency, since the radiant heat from the porous sheets which arehigher temperature parts is reflected by the reflective cooling platemembers which are lower temperature parts. Also, since the temperatureelevation of the reflective cooling plate members can be avoided by thereflection of the radiant heat (the reflective cooling plate membersobtain only the latent heat of the condensation of the water vapor), thedifference in temperature between the porous sheet and the cooling platemember facing each other can be maintained high, and as a result, themist-like water vapor particles which have a specific gravity of 1 andis heavier than air, receive a force of moving from the porous sheet tothe reflective cooling plate member by the separation action of thethermal diffusion effect, and quickly move to the cooling plate memberand is condensed and separated on the reflective surface thereof toproduce distilled water in an improved production efficiency.

The term "separation action of the thermal diffusion effect" as usedherein means a diffusion phenomenon in which a lighter component in amixed fluid of two components is attracted to a higher temperature sideand a heavier component is attracted to a lower temperature side.

In a mixed gas of two components, thermal diffusion ratio k_(T) is shownby the equation:

    k.sub.T =-dC.sub.1 /d(log T)=dC.sub.2 /d(log T)

wherein C₁ is a molar fraction of a heavier component, C₂ is a molarfraction of a lighter component and T is absolute temperature, and thethermal diffusion has been utilized in separation of isotopes. In thepresent invention, a higher temperature portion (i.e. the porouswater-absorptive sheet attached to the heat receiving plate member orthe reflective cooling plate member) and a lower temperature portion(i.e. the reflective surface of the reflective cooling plate member) areopposite to each other and between them heavier fine liquid waterparticles and lighter molecules of air are present. Therefore, while thewater vapor and air which are higher temperature and the liquid waterparticles and air which are lower temperature circulate upward anddownward, respectively, by convection, simultaneously the liquid waterparticles are attracted toward the reflective cooling plate member andquickly condense on the reflective surface by the separation action ofthe thermal diffusion effect.

Since the process of the present invention is conducted by the multieffect system, the solar heat can be efficiently utilized. Of course,the process may be conducted by a system of one effect, that is, byemploying a combination of a heat receiving plate member and a singlereflective cooling plate member. If there is no heat loss, anapproximately equal amount of distilled water is produced in each effectby a certain amount of solar energy and it is theoretically possible toinfinitely produce distilled water. However, in practical production aproduction speed is required and, therefore, it is desired to maintainthe difference in temperature between the higher temperature side (i.e.the porous water-absorptive sheet) and the lower temperature side (i.e.the reflective surface of the cooling plate member) facing each otherabout 5° to about 15° C. For instance, when the temperature of the heatreceiving plate member is 90° C., and the temperature of the outerreflective cooling plate member is 30° C., it is practical to conductthe process by a system of 3 to 12 effects.

FIG. 1 is a longitudinal section view of a vertical type solardistillation apparatus showing an embodiment of the present invention inwhich two transparent covers for keeping warmth and three reflectivecooling plate members are employed, and FIG. 2 is a partially cutwayperspective view of the apparatus shown in FIG. 1. Numerals 11 and 12are transparent covers for keeping warmth through which the sunlighttransmits and falls on a heat receiving plate 1. Both ends of the covers11 and 12 are attached to heat insulating frames 9 and 10. Thetransparent cover is located outside the heat receiving plate 1 in anappropriate space therefrom so as to prevent the heat energy loss of theplate 1 heated by the sunlight, and as shown in FIG. 1 the double coveris preferred than the single cover. The suitable materials as thetransparent cover are a transparent glass plate and a transparentplastic board, sheet or film. Numerals 13 and 14 are upper and lowertanks of overflow type, and they are attached to the heat insulatingframes 9 and 10, respectively. Porous water-absorptive sheets 2, 4, 6and 8 are stuck to the reverse sides of the heat receiving plate 1 andreflective cooling plates 3, 5 and 7. The cooling plates 3, 5 and 7 arelocated inside the heat receiving plate 1, and are arranged in parallelspaced relationship with the heat receiving plate 1 and with each other.The upper portions of the cooling plates to which the porous sheets arestuck are integrated in layers, and the integrated upper portions areinserted into an opening of the upper tank 13 so that the ends of theporous sheets are immersed into a water 16 to be treated in the tank 13.The lower end of the heat receiving plate 1 is attached to the frame 10.The lower ends of the cooling plates 3, 5 and 7 are in L-shape and areattached to the frame 10 so as to form gutters 17, 18 and 19 forcollecting the condensed water. It is desirable that the gutter has thewidth corresponding to the space between the porous sheet and thecooling plate. The ends of the porous sheets 2, 4, 6 and 8 extendthrough the heat insulating frame 10 to the lower tank 14 attachedoutside the frame 10 so as to be immersed into a water 15 to be treatedin the tank 14.

The heat receiving plate 1 has a surface capable of absorbing radiantheat of sun. The heat receiving plate receives the sunlight and convertsit into the heat energy so as to evaporate water absorbed by the poroussheet stuck onto the back of the heat receiving plate. For instance, asthe heat receiving plate 1 there may be employed thin corrosionresistant metal plates (thickness of which is usually about 0.05 toabout 1 mm.) having a black or dark heat receiving coating capable ofselectively absorbing the solar heat rays in the wave length region ofabout 0.3 to about 2.5μ. Such a heat receiving coating is formed byapplying a coating composition containing a synthetic resin such as analkyd resin, an urethane resin, an epoxy resin, a fluorinecontainingresin or an acrylic resin and a filler such as carbon black, potassiumchromate or strontium chromate onto the surface of the corrosionresistant metal plate such as stainless steel or aluminum. Also, atransparent plastic sheet or film is usable as the heat recivingplate 1. In that case, a black porous water-absorptive sheet is stuck tothe back of the plastic sheet so that the sunlight transmits through theplastic sheet and directly hits the black porous water-absorptive sheet.

Although the solar heat absorbed by the heat receiving plate 1 isconsumed to evaporate water, there may occur heat losses due to theconvection of air, thermal radiation and thermal conduction by materialsof apparatus. These heat losses raise the temperature of a lowertemperature part and prevents the condensation of water vapor so as tolower the production yield of distilled water, though the convection ofair may accelerate the evaporation of water. The heat loss due to theradiation is relatively large, and in order to prevent this heat loss acooling plate having a reflective surface is preferably employed. Sincethe reflective surface of the cooling plate reflects 90 to 95% of theradiant heat and can prevent the heat loss due to the radiation, theyield of the distilled water can be remarkably increased.

The reflective cooling plate is required to have the following threefunctions. That is to say, it is necessary that the cooling plate makesthe vapor water generated at a higher temperature part condense on thereflective surface and makes the condensed liquid water flow downward asdistilled water, that the cooling plate reflects the radiant heat from ahigher temperature part opposite thereto so as to prevent the cooling ofthe higher temperature part, in other words, so as to prevent the heatloss by radiation, and that the cooling plate receives the latent heatreleased by the condensation of water vapor and quickly conducts it tothe porous water-absorptive sheet stuck to the back so as to heat andevaporate the water absorbed by the porous sheet. Suitable materials forthe reflective cooling plate are metal plates having a good corrosionresistance and a good thermal conductivity such as stainless steel andaluminum and transparent plastic sheets or films having a vacuummetallized surface such as aluminum or silver. The thickness of themetal plate is usually from about 0.05 to about 1 mm., and it isdesirable that the metal plate is finished to give a mirror surface or asmooth surface. Also, it is desirable that the vacuum metallized surfaceof the plastic sheet or film is protected with a transparent plasticcoating to provide a water resistance and corrosion resistance. Acorrugated reflective cooling plate may be employed for the purpose ofincreasing a heat absorption area, evaporation area, cooling area andstrength.

As the porous water-absorptive sheet to be stuck to the back of the heatreceiving plate and the back of the reflective cooling plate, there areemployed those capable of drawing up a water to be treated by capillaryaction, e.g. hydrophilic felts, plastic foams having continuous bubbles,cloths and papers. A felt, foam and cloth made of polyvinyl formal havea good water-absorptivity, and are suitably employed in the presentinvention. Also, it is desirable that the porous water-absorptive sheetis durable for use for a long term without being changed in quality by awater to be treated such as sea water or a waste water and moreover hasa good thermal conductivity. There are also employed papers, cloths andmats made of carbon fiber, glass fiber, an anti-corrosive metal fibersuch as stainless steel, asbestos fiber or a mixture of these fibers aswell as the above-mentioned materials.

The distances between the heat receiving plate and the cooling plate andbetween two adjacent cooling plates are usually selected from 2 to 50mm., and thereby a good thermal diffusion effect can be attained. Also,the distance between the covers when two covers are used and between thecover and the heat receiving plate are usually selected from 10 to 30mm.

A water 16 to be treated is supplied through a feed pipe 20 to the uppertank 13, while overflowing through an overflow pipe 21, and iscontinuously absorbed by the porous water-absorptive sheets 2, 4, 6 and8 stuck to the reverse sides of the heat receiving plate 1 and thereflective cooling plates 3, 5 and 7. Direct and scattering rays of thesun from the sky and reflected sunlight from the ground or the surfaceof the sea transmit through the transparent covers 11 and 12 and fall onthe heat receiving plate 1, and the solar heat is absorbed thereby. Thesunlight hitting the dark surface causes it to heat and evaporates thewater absorbed by the porous sheet 2 stuck to the back of the heatreceiving plate 1. The space between the porous sheet 2 and the adjacentcooling plate 3 opposite thereto is filled with the water vapor in thestate of saturation. Since the temperature of the cooling plate 3 islower than that of the porous sheet 2 stuck to the heat receiving plate1, a part of the saturated water vapor is cooled below the dew point,and becomes very fine mist-like water particles which are then drawntoward to reflective surface of the cooling plate 3 by the thermaldiffusion action and condensed on the reflective surface. The dewdropsrun down and are collected as distilled water by the collecting gutter17. Simultaneously the reflective surface of the cooling plate reflectsthe radiant heat from the porous sheet facing the reflective surface toprevent the heat loss by radiation.

The reflective cooling plate 3 obtains the latent heat of thecondensation, and heats and evaporates the water absorbed by the poroussheet 4 stuck to the back of the cooling plate 3. The water vaporcondenses on the reflective surface of the cooling plate 5 and the waterdroplets run down to the collecting gutter 18 in the same manner as inthe first effect partitioned by the heat receiving plate 1 and thereflective cooling plate 3. The evaporation of water by the latent heatand the condensation of water vapor are repeated in each effectpartitioned by two reflective cooling plates according to the number ofthe effects. The reflective cooling plate in the last effect is alwaysmaintained low temperature. That is to say, in FIGS. 1 and 2, the waterfor cooling absorbed by the porous sheet 8 stuck to the back of thereflective cooling plate 7 is evaporated by means of the outside air,and the cooling plate 7 is sufficiently cooled by the heat ofvaporization. This also makes the reflective cooling plates 3, 5 and 7maintain a constant temperature difference from each other, and they arein equilibrium state.

Although the number of the effects may be increased by employing a largenumber of the reflective cooling plates, there is a certain limit in thenumber of the cooling plates, since the yield of distilled water isreduced when the temperature difference between the adjacent tworeflective cooling plates becomes too small.

When a water to be treated is sea water, sea water contained in theporous sheets concentrates and the concentration of salt increases withthe evaporation of sea water. However, the salt naturally diffuses intothe sea water flowing in the upper and lower tanks 13 and 14 not so asto increase over a certain concentration, and is maintained inequilibrium state.

The water supply to the porous sheets may be conducted from both theupper and lower tanks 13 and 14, or from either the upper tank 13 or thelower tank 14 by the capillary action. In case that the height of theapparatus is large and sufficient supply of a water to be treated, e.g.sea water, from the lower tank is difficult, the water may be suppliedfrom only the upper tank 13 and the concentrated water may be dischargedto the lower tank 14. In that case, since the water discharged to thetank 14 is hot, it may be employed to increase the heat efficiency bypassing it from an overflow pipe 22 into a heat exchanger (not shown)and supplying a feed water heated by the heat exchanger to the uppertank 13 through the pipe 20. Also, it is possible to pre-heat the feedwater by means of a heater such as a solar heater or by utilizing otherwaste heat. When the pre-heated water is supplied to the apparatus, theyield of distilled water approaches the theoretical value and,therefore, it is possible to increase the amount of the produceddistilled water according to the increased number of the reflectivecooling plates, in other words, of the effects. The solar heat absorbedby the heat receiving plate causes water to evaporate, and while thewater vapor condenses on the reflective surface to produce distilledwater, it causes water held on the back of the cooling plate toevaporate so as to produce on the reflective surface of the next coolingplate an approximately equal amount of distilled water to the distilledwater produced on the preceding reflective cooling plate. On thesucceeding every reflective cooling plate, an approximately equal amountof distilled water to the distilled water produced on the precedingcooling plate is produced in the same manner.

FIGS. 3 and 4 show a solar distillation apparatus of inclination typeaccording to the present invention. The transparent covers 11 and 12,heat receiving plate 1 and reflective cooling plates 3, 5 and 7 arearranged in that order in parallel with each other, and both ends of thecovers 11 and 12 and the heat receiving plate 1 are attached to the heatinsulating side walls 9 and 10. The upper ends of the porouswater-absorptive sheets 2, 4, 6 and 8 stuck to the reverse sides of theplates 1, 3, 5 and 7 are connected to a porous water-absorptive thicksheet 30 for distributing a water to be treated which is provided insidethe side wall 9, and the lower ends thereof are connected to a porouswater-absorptive thick sheet 40 for recovering the distilled water whichis provided inside the side wall 10. A water 16 to be treated issupplied from a feed pipe 20 to the distributing porous thick sheet 30through a pipe 20a, and is evaporated from the porous sheets 2, 4, 6 and8 with moving downward, while it is continuously supplied according tothe evaporated amount. The water vapor condenses on the reflectivesurface of the cooling plates 3, 5 and 7. The condensed water runs downto the bottom portions 17, 18 and 19 of the reflective cooling platesalong the reflective surfaces, and is collected as pure water throughpipes 23a, 23b and 23c provided at the bottom portions 17, 18 and 19 andthen through a pipe 23. The supplied excess water permeates into therecovering porous thick sheet 40 connected with the lower ends of theporous sheets 2, 4, 6 and 8, and is led to a pipe 22 through a pipe 22a.

In FIG. 5 wherein the reference numerals are the same as those in FIGS.1 and 2 and the reference numeral 23 is a pipe for recovering distilledwater, two vertical type solar distillation apparatuses as shown inFIGS. 1 and 2 are installed in parallel spaced relationship with eachother, and the back side of one apparatus and the back side of the otherapparatus stand opposite to each other. A reflector may be employed toincrease the yield of distilled water, since the heat receiving platepositioned at the shady side is exposed to the scattering rays of thesun, but not exposed to the direct rays of the sun.

FIG. 6 shows a cylindrical solar distillation apparatus according to thepresent invention, the body of which is a round shape in section. Thetransparent covers 11 and 12, heat receiving plate 1 and reflectivecooling plates 3, 5 and 7 are concentrically arranged in that order fromthe outside, and the bottoms thereof are fixed to a heat inslating base10. The porous water-absorptive sheets 2, 4, 6 and 8 are stuck to theinner sides of the heat receiving plate 1 and the cooling plates 3, 5and 7. A water to be treated is supplied to the upper tank through thefeed pipe. Gutters 17, 18 and 19 are provided at the bottoms of and infront of the reflective cooling plates 3, 5 and 7, and the condensedwater is collected through the pipes 23a, 23b and 23c connected to thegutters 17, 18 and 19 and is recovered from the pipe 23 connected to thepipes 23a, 23b and 23c. The numeral 22 is an overflow pipe which isconnected to the lower tank (not shown). The apparatus of this type iscompact and can be easily set up, and also has the advantages that asmall area for setting up is sufficient and the efficiency is good. Thedirect and scattering rays of the sun fall onto the heat receiving platefrom all directions and causes water to evaporate, whereas thecyrindrical space at the center portion (to which the porous sheet 8 isexposed) is cooled by means of air cooling or water cooling, sodistilled water can be produced in a high yield.

The solar distillation apparatus of the present invention is suitablyemployed for desalting sea water or for producing water fit to drink orwater for agriculture from an industrial waste water, a domestic wastewater or an underground or liver water containing salts or impurities.

The solar distillation apparatus of the present invention can producedistilled water in an amount of more than 10 times that produced by aconventional basin type solar distillation apparatus with respect to anequal amount of solar energy, and the heat efficiency is very high. Theyield of distilled water may be further increased by compulsorilyblowing the outside air to the porous water-absorptive sheet which isstuck to the back of the last reflective cooling plate and is exposed tothe outside air so as to be cooled by the contact with the outside airor the evaporation of water. In case of desalting sea water on the sea,the efficiency may be further increased, when a cold sea water drawnfrom the sea bottom is used as a cooling water.

In addition to the solar heat, it is possible to utilize a waste heat asthe heat source by bringing the heat of the earth or a hot water or airof a relatively low temperature discharged from a power plant or afactory into contact with the heat receiving plate, and new waterresources may be developed by effectively utilizing a waste energy.

Another embodiment of the present invention is a convection type multieffect distillation apparatus as shown in FIGS. 7 to 9. In the apparatusof this type, a plurality of shallow dish-like vessels, e.g. severalpieces to several tens pieces of the vessels, are put in layers to forma multi stage structure body. While a water to be treated is held in thevessels, the under surfaces of the bottoms of the vessels act as acooling plate member and the water vapor is condensed thereon. The watersupplied from a water supply means to the top vessel overflows to thenext under vessel and eventually reaches the lowest vessel throughoverflow pipes attached to the vessels. The water in the lowest vesselis heated, for instance, to near boiling temperature, by a heating meanssuch as a heater attached to the under surface of the lowest vessel incontact therewith or by means of the sunlight reflected or gathered toheat the bottom of the lowest vessel which acts as the heat receivingplate member. The water vapor generated in the lowest vessel iscondensed on the under surface of the adjacent upper vessel, whilegiving the latent heat of the condensation to the bottom of the adjacentupper vessel and causing the water in the adjacent upper vessel to heatand evaporate so as to generate the water vapor corresponding to thevapor pressure at the temperature of the heated water. This water vaporwas then condensed on the under surface of the adjacent upper vessel,while giving the latent heat of the condensation to the bottom of theadjacent upper vessel. Such evaporation and condensation are repeated ineach effect partitioned by a lower vessel and an upper vessel. The heatgiven to the lowest vessel transfers in order from a lower vessel to anupper vessel by convection in the upward direction, in other words, in acontrary direction of the flow of the water to be treated. Thetemperature of the water in each effect is in a stationary state. Ifthere is no heat loss, it is theoretically possible to infinitelyproduce distilled water by a certain limited heat energy. However, inpractical production a production speed is required and, therefore, itis desired to maintain the difference in temperature between a highertemperature part (i.e. a water to be treated in a lower vessel) and alower temperature part (i.e. a water to be treated in the next uppervessel) usually by about 4° to about 10° C. For instance, when thetemperature of the water in the lowest vessel is 95° C. and thetemperature of the water in the top vessel is 55° C., it is practical toconduct the process by a system of 5 to 10 effects.

The condensed water on the under surface of the bottom of each vessel istaken out as distilled water. The top vessel may be covered with acooling plate cooled by means of air cooling so that the water vaporevaporated from the top vessel is condensed on the under surface of thecooling plate as distilled water which is then taken out. Of course, thetop vessel filled with water may be employed for cooling the water vaporgenerated from the adjacent lower vessel. U-shaped packings areinterposed between respective adjacent two vessels so as to form slitopenings for taking out the condensed water which is then collected intoa pure water gutter. Although the under surface of the bottom of thedish-like vessel may be horizontal, it is preferably inclined at anangle of 2° to 10°, particularly 3° to 5° with respect to a horizontalplane, since the condensed water is made easily flow out along the undersurface. This may be attained by means of the packings interposedbetween the vessels, the formation of the bottom of the vessel in theinclined plane or installation of the apparatus with a gradient.

In either case of making the under surface of the bottom of the vesselhorizontal or inclining it, porous water-absorptive sheets may be stuckto all over the under surface of each bottom of the vessels, and bywhich the condensed water is absorbed without dropping into the adjacentlower vessel. Also, it is desirable that the porous sheets extendthrough the slit openings to the outside of the evaporation system,preferably to the pure water collecting gutter located below the slitopening of the lowest vessel. The condensed water absorbed by the poroussheets flows out from the evaporation system by capillary action andthen runs down to the gutter by capillary action and siphon action.Since the condensed water absorbed by the porous sheet at the undersurface of the bottom of the upper vessel forms a water layer, itbecomes a good conductor of heat and the water vapor condenses veryeasily on the surface of the porous sheet, while giving the latent heatof the condensation to the next upper vessel so as to heat and evaporatethe water. Examples of the porous water-absorptive sheet are a cloth orfelt made of a hydrophilic fiber such as polyvinyl formal fiber,polyamide fiber or cellulose fiber, a cloth or non-woven fabric made ofglass fiber, carbon fiber or asbestos fiber, and a hydrophilic sheetmade of a synthetic resin foam such as urethane foam incorporated withgraphite. A cloth and felt made of a hydrophilic fiber are preferred.The thickness of the porous sheet is selected from 0.1 to 10 mm.,preferably 1 to 3 mm. according to the area of the condensing surfaceand the amount of the condensed water. The porous sheet may be stuck tothe under surface of the bottom of the vessel by applying an adhesivehaving a water resistance and a heat resistance such as an acrylicadhesive or an epoxy adhesive to all over the under surface, to theunder surface at regular intervals, or to the corners of the undersurface and sticking the porous sheet thereto. The adhesive is desirablyapplied to the under surface at regular intervals or to the corners inviewpoint of the heat conductivity. In case that the vessel having acorrugated bottom is employed, the adhesive is desirably applied to thedepressed portions of the corrugated under surface.

The size of the dish-like vessel is usually 300 to 1,000 mm. in lengthand 500 to 2,000 mm. in width. The height of the vessel is preferablyselected from the range of 10 to 100 mm., and thereby the concentrationof a water to be treated is maintained constant and also the temperaturedifference between a lower vessel and the adjacent upper vessel is madeas small as possible and the heat transfer and the production of thecondensed water are increased so that the number of the stages may beincreased and the heat efficiency may be increased. The suitablematerials for the dish-like vessel are a coated metal such as aluminum,cupper, iron or steel, and an anti-corrosive metal such as stainlesssteel or titanium. The anti-corrosive stainless steel and titanium andan alloy thereof are preferably employed. The depth of the water held inthe vessel is usually 10 to 50 mm., and accordingly an overflow pipe isattached to the vessel so that the depth of the water falls within theabove range.

The slit opening for taking out the condensed water is formed byinterposing the U-shaped packing between the adjacent two vessels. Thewidth of the slit opening in the longitudinal direction is from 1 to 10mm., preferably 2 to 3 mm. In case of employing the porouswater-absorptive sheet, the longitudinal width of the slit opening isadjusted by changing the thickness of the packing with taking thethickness of the porous sheet into consideration.

The feed amount of a water to be treated is adjusted so that the wateris discharged from the overflow pipe attached to the lowest vessel in anamount of 1 to 3 liters/hours/m². Suitable feed amount is about 2 toabout 3 times the amount of the distilled water collected, and with thesupply of the water in such a ratio the pure water is produced in a goodheat efficiency without causing the deposit of a scale.

Supposing that the amount of the heat given to the water to be treatedin the lowest vessel is Q calories and there is no heat loss, all thegive heat Q is consumed by the evaporation of the water. Supposing thatthe latent heat of the condensation of the water vapor is Lcalories/kg., the amount of the distilled water produced in each effectis Q/L kg. If the number of the stages, i.e. the effects, of theapparatus is n, total amount of the produced distilled water is Q/L×nkg. However, in practice, a part of the heat is taken away by thecondensed water and the discharged overflowing water from the lowestvessel and is also lost by the heat conduction through the apparatusmaterials. Therefore, in order to decrease the heat loss and toefficiently produce distilled water, the multi stage structure body ispreferably covered with a heat insulating material. Also, it isdesirable to recover the heat of the condensed water and the dischargedwater by means of a heat exchanger so as to heat the supplied water tobe treated.

Now referring to FIG. 7 showing a convection type multi effectdistillation apparatus according to the present invention wherein porouswater-absorptive sheets are stuck to the under surfaces of the bottomsof dish-like vessels and they extend to a pure water collecting gutterthrough slit openings, the numerals a₁, a₂, a₃, a₄, a₅ . . . a_(n-2),a_(n-1) and a_(n) are dish-like vessels for holding a water to betreated and they are horizontally piled up in the state that U-shapedpackings S₁, S₂, S₃, S₄, S₅ . . . S_(n-2) and S_(n-1) are interposedbetween them to form slit openings G₀, G₁, G₂, G₃, G₄, G₅ . . . G_(n-2)and G_(n-1). Porous water-absorptive sheets F₁, F₂, F₃, F₄, F₅ . . .F_(n-2) and F_(n-1) are adhered with an adhesive to the under surfacesof the bottoms of the vessels, and extend through the slit openings to acondensed water collecting gutter 53 located below the slit openingG_(n-1) of the lowest vessel a_(n) so as to immerse in a distilled water52 collected in the gutter 53. Three side walls except the side wherethe slit openings are located, which are formed by the side walls of thevessels and the packings, are covered with a heat insulating material10, and the slit opening side is covered with the heat insulatingmaterial 10 with leaving a space between them so that the porouswater-absorptive sheets hang down to the gutter 53 located at the bottomof the space.

A water to be treated is continuously supplied from a water feed pipe 51to the top vessel a₁ in a prescribed amount, and the water b₁ filed inthe vessel a₁ overflows through an overflow pipe P₁ into the vessel a₂.In the same manner, waters b₂, b₃, b₄, b₅ . . . b_(n-2) and b_(n-1) inthe vessels a₂, a₃, a₄, a₅ . . . a_(n-2) and a_(n-1) overflow throughoverflow pipes P₂, P₃, P₄, P₅ . . . P_(n-2) and P_(n-1) to respectivenext lower vessels, and water b_(n) in the lowest vessel a_(n) isdischarged through an overflow pipe P_(n). The overflow pipes areattached to the bottoms of the vessels and project through the bottom toa desired height. The overflow pipes of the adjacent two vessels locateat the end portions of the sides different from each other. The waterb_(n) held in the lowest vessel a_(n) is heated to about 100° C. by aheater 55 attached to the bottom of the vessel a_(n) in contact with theunder surface of the bottom. Electric heat, a fuel, steam, a waste heatand a hot water are usable as a heat source for the heater 55. Theresulting water vapor condenses on the porous sheet F_(n-1) stuck to theunder surface of the bottom of the next upper vessel a_(n-1), andsimultaneously release the latent heat of the condensation so as to heatthe water b_(n-1) in the vessel a_(n-1). The liquid water condensed onthe porous sheet F_(n-1) is absorbed by the porous sheet F_(n-1), andmoves to the outside of the evaporation system and runs down to thecondensed water collecting gutter 53 by capillary and siphon actions.The distilled water 52 in the gutter 53 is taken out through a pipe 54.The water b_(n-1) in the vessel a_(n-1) heated by the latent heat of thecondensation generates the water vapor, and the water vapor condenses onthe porous sheet F_(n-2) stuck to the under surface of the bottom of thenext upper vessel a_(n-2) with heating the water b_(n-2) in the vessela_(n-2) by the latent heat of the condensation. In the same manner, heatexchange is conducted in order between the water vapor generated fromthe water to be treated in a lower vessel and the water to be treated inthe adjacent upper vessel to produce distilled water. The water vaporgenerated from the water b₁ in the top vessel a₁ condenses on porouswater-absorptive sheet F₀ stuck to a cooling plate 56 which issuperposed on the top vessel a₁ with interposition of a packing S₀. Thelatent heat released at that time radiates to air. The waters condensedon the porous sheets F_(n-1), F_(n-2) . . . F₅, F₄, F₃, F₂, F₁ and F₀are absorbed by the porous sheets, and run down to the gutter 53 throughthe porous sheets by capillary and siphone actions. The collecteddistilled water 52 is taken out through the pipe 54 and stored in a tank(not shown).

FIG. 8 is a partially cutway perspective view of a convection type multieffect distillation apparatus showing the state that the apparatus isdisassembled. Six shallow dish-like vessels a₀, a₁, a₂, a₃, a₄ and a₅are horizontally piled up with interposition of U-shaped packings S₀,S₁, S₂, S₃ and S₄ to form an apparatus body. Porous water-absorptivesheets F₀, F₁, F₂, F₃ and F₄ are stuck with an adhesive to therespective under surfaces of the bottoms of the vessels. Slit openingsG₀, G₁, G₂, G₃ and G₄ are formed on one side of the body by theinterposition of the U-shaped packings, and through which the poroussheets stuck to the under surface of the vessels extend and hang down toa condensed water collecting gutter 53 positioned below the slit openingG₄ of the lowest vessel a₅. The distilled water collected in the gutter53 is taken out through a pipe 54. The reference numerals P₀, P₁ and P₅are overflow pipes attached to the bottoms of the vessels a₀, a₁ and a₅through holes provided at the bottoms. The four sides of the apparatusbody are covered with a heat insulating material 10.

FIG. 9 is a longitudinal section view of an embodiment of the convectiontype multi effect distillation apparatus according to the presentinvention, in which the sunlight is employed as a heat source and theapparatus is provided with a sunlight gathering box as a heater underthe lowest dish-like vessel and an inclined transparent glass plate as acover operating as a basin type evaporator over the top dish-likevessel. The sunlight gathering box 70 is made of aluminum mirror plates71 having a good reflectivity to provide an reflective inner wallsurface. The bottom of the box 70 is finished to a spherical surface, aparabolic surface of the combined surface thereof, and the inner sidewalls of the box 70 is finished to a flat surface. The sunlighttransmitting through a transparent glass window 72 is gathered to theunder surface 75 of the bottom of the lowest vessel a₇ by the mirrorplates 71. The under surface 75 of the bottom of the lowest vessel a₇ isa surface capable of absorbing the solar heat rays, and a solar heatabsorptive coating is usually applied to the under surface 75. Since thecoating absorbs most of the heat of the gathered sunlight and scarcelyemits radiant rays of itself, the efficiency of absorbing heat is verygood. The solar heat absorptive coating is usually formed by applying ablack or dark paint of a synthetic resin such as an alkyd resin, aurethane resin, an epoxy resin or an acrylic resin incorporated with afiller such as carbon black, potassium chromate or strontium chromate tothe under surface 75 of the bottom of the lowest vessel a₇. Such acoating can selectively absorb the solar heat rays within the wave rangeof 0.3 to 2.5μ.

Dish-like vessels a₀, a₁, a₂, a₃, a₄, a₅, a₆ are horizontally put instages with interposition of U-shaped packings S₀, S₁, S₂, S₃, S₄, S₅and S₆. Porous water-absorptive sheets F₀, F₁, F₂, F₃, F₄, F₅ and F₆ arestuck with an adhesive to the under surface of the bottoms of thevessels, and extend through slit openings G₀, G₁, G₂, G₃, G₄, G₅ and G₆formed by the interposition of the packings between the vessels to acondensed water collecting gutter 53 located below the slit opening G₆of the lowest vessel a₇. The water vapor generated in each vesselcondenses on the porous sheet stuck to the under surface of each vessel,and is collected into the gutter 53. Overflow pipes P₀, P₁, P₂, P₃, P₄,P₅ and P₆ are attached to the bottoms of the vessels through holes madein the bottoms. An overflow pipe P₇ is attached to the side wall of thevessel a₇, and a water to be treated overflowing from the overflow pipeP₇ is introduced to a usual contact type heat exchanger 60 and isdischarged through a pipe 65.

An inclined cover 73 of a transparent glass plate is provided over thetop vessel a₀ to form a basin type evaporator. The sunlight transmittingthrough the transparent cover 73 causes a water supplied into the topvessel a₀ from a water feed pipe 51 to heat and evaporate, and the watervapor condenses on the under surface of the cover 73. The condensedwater runs down along the under surface of the cover 73 and is collectedinto a condensed water collecting gutter 74.

A water to be treated is first supplied to a water feed pipe 51a and ispreheated by the heat exchanger 60 in which the heat exchange isconducted between the water of a high temperature overflowing from thepipe P₇ and the supplied water. The preheated water is further preheatedby a hot distilled water collected in a condensed water collectinggutter 53 which operates as a heat exchanger, and then supplied throughthe feed pipe 51 to the top vessel a₀ which operates as a basin typeevaporator. In the heat exchanger 60, a water of a high temperaturedischarged from the overflow pipe P₇ runs down along a felt 61 forevaporation with generating the water vapor, and the water which hasgrown cold by the evaporation is collected in a drainage collectingvessel 63 and is discharged from a pipe 65. The water vapor generatedfrom the felt 61 is cooled by the feed pipe 51a through which a water tobe treated passes, and is condensed on a condensing surface 62. Thecondensed water is collected into a condensed water collecting vessel 64and is taken out as distilled water from a pipe 54.

The convection type multi effect distillation apparatus according to thepresent invention is very simple in structure, and also does not requireenergy for operating the apparatus, since the gravity due to a head isutilized for the transportation of a liquid and since the operation isconducted at atmospheric pressure and there is no need for reducingpressure. Further, the heat conduction, heat exchange and condensationof the water vapor are effectively conducted based on the multi stagestructure of the piled vessels without requiring a particular device.Therefore, since there is no moving portion and the apparatus is offixing type, no trouble takes place. Moreover, the heat efficiency isvery high, and for instance, it is possible to attain the heatefficiency of 20 times that of a conventional basin type solardistillation apparatus. Therefore, distilled water can be produced in anamount of about 5 to about 20 times that produced by a conventionaldistillation apparatus with respect to an equal amount of heat energy.

The present invention is more particularly described and explained bymeans of the following Examples. These examples are intended toillustrate the invention and not to be construed to limit the scope ofthe invention.

EXAMPLE 1

Distillation of sea water was carried out by employing a thermaldiffusion type vertical solar distillation apparatus in which a singlereflective cooling plate is employed. That is to say, the employedapparatus was made up of a heat receiving plate to the back of which aporous water-absorptive sheet was stuck, a reflective cooling plate tothe back of which the porous sheet was stuck, a heat insulating frame,upper and lower tanks, a transparent double cover and a condensed watercollecting gutter. A stainless steel plate having a thickness of 0.3 mm.was employed as the heat receiving plate and the reflective coolingplate, and a polyvinyl formal foam sheet (thickness: 1 mm., apparentspecific gravity: 0.12 g/cm.³, porosity: 91%) was employed as the poroussheet. The heat receiving plate had a black coating (absorption ratio:97 to 98%), and the area of the heat receiving surface was 1.5 m.² (1.5m. long and 1.0 m. broad). The surface of the cooling plate was smooth,and the reflectivity of the reflective surface was 90%. The area of thereflective surface was 1.4 m.² (1.4 m. long and 1.0 m. broad). Atransparent sheet glass having a thickness of 3 mm. was employed as thecovers.

The weather upon the distillation was fine, and the test was carried outunder the following conditions: atmospheric temperature 23° to 27° C.,relative humidity 72%, amount of solar radiation 598 kcal./m.².hour andwind velocity 2 to 4 m./sec. Sea water was supplied from a feed pipe tothe upper tank with overflowing from an overflow pipe connected to theupper tank. Sea water which had run down through the porous sheets stuckto the heat receiving plate and the cooling plate and reached the lowertank was discharged from the lower tank. The temperature of the seawater which reached the lower tank through the porous sheet of the heatreceiving plate was 69° C., and the temperature of the sea water whichreached the lower tank through the porous sheet of the cooling plate was40° C. The amount of the discharged sea water was 1.4 liters/hour.

The results are shown in Table 1, in which the heat efficiency iscalculated by the following equation and hereinafter means the same.##EQU1##

                  TABLE 1                                                         ______________________________________                                        Temperature of heat receiving plate                                                                  83° C.                                          Temperature of reflective cooling plate                                                              56° C.                                          Amount of produced distilled water                                                                   1,131 ml./hour                                         Heat efficiency        71%                                                    ______________________________________                                    

EXAMPLE 2

Distillation of sea water was carried out by employing a thermaldiffusion type vertical solar distillation apparatus as shown in FIG. 1.

Stainless steel plates (thickness: 0.3 mm.) were employed as the heatreceiving plate 1 and the reflective cooling plates 3, 5 and 7. The heatreceiving plate 1 had a black coating (absorption ratio: 97 to 98%), andthe heat receiving surface area was 1.5 m.² (1.5 m. long and 1.0 m.broad). The cooling plates 3, 5 and 7 had smooth surface (reflectivity:90%), and the reflective surface area of each plate was 1.4 m.² (1.4 m.long and 1.0 m. broad). Polyvinyl formal foam sheets (thickness: 1.0mm., apparent specific gravity: 0.12 g./cm.³, porosity: 91%) wereemployed as the porous water-absorptive sheets 2, 4, 6 and 8. Also,transparent glass sheets (thickness: 3 mm.) were employed as thetransparent covers 11 and 12.

The weather at the time of distillation was fine, and the test wascarried out under the following conditions: atmospheric temperature 23°to 26° C., relative humidity 75%, amount of solar radiation 603kcal./m.².hour and wind velocity 1 to 4 m./sec. Sea water was suppliedfrom the feed pipe 20 to the upper tank 13 with overflowing from theoverflow pipe 21. Sea waters which had run down through the poroussheets 2, 4, 6 and 8 and reached the lower tank 14 was discharged fromthe lower tank 14. The temperatures of the sea waters which reached thelower tank 14 through the porous sheets 2, 4, 6 and 8 were 83° C., 71°C., 54° C. and 42° C., respectively. The amount of the discharged seawater was 2.1 liters/hours.

The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Temperature:                                                                  Heat receiving plate 1                                                                            97° C.                                             Reflective cooling plate 3                                                                        80° C.                                             Reflective cooling plate 5                                                                        64° C.                                             Reflective cooling plate 7                                                                        46° C.                                             Amount of distilled water produced:                                           Between the plates 1 and 3                                                                        1,175 ml./hour                                            Between the plates 3 and 5                                                                          893 ml./hour                                            Between the plates 5 and 7                                                                          681 ml./hour                                            Total               2,749 ml./hour                                            Heat efficiency     170%                                                      ______________________________________                                    

EXAMPLE 3

Distillation of sea water was carried out by employing a thermaldiffusion type inclined solar distillation apparatus as shown in FIG. 3.

Materials of the apparatus were the same as those of the verticaldistillation apparatus employed in Example 2 except that the heatreceiving surface area of the heat receiving plate 1 was 2.0 m.² (1.0 m.long and 2.0 m. broad) and the reflective surface area of each of thereflective cooling plates 3, 5 and 7 was 2.0 m.² (1.0 m. long and 2.0 m.broad).

The weather upon the distillation was fine, and the test was carried outunder the following conditions: atmospheric temperature 19° to 25° C.,relative humidity 78%, amount of solar radiation 595 kcal./m.².hour andwind velocity 1 to 3 m./sec. Sea water was supplied from the feed pipe20a to the distributing porous water-absorptive thick sheet 30, and thesea water which had passed through the porous water-absorptive sheets 2,4, 6 and 8 was collected by the recovering porous water-absorptive thicksheet 40 and was discharged through the pipes 22a and 22. Thetemperatures of the sea waters at the lower ends of the porous sheets 2,4, 6 and 8 were 86° C., 74° C., 58° C. and 43° C., respectively. Theamount of the discharged sea water was 2.3 liters/hour.

The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Temperature:                                                                  Heat receiving plate 1                                                                            98° C.                                             Reflective cooling plate 3                                                                        84° C.                                             Reflective cooling plate 5                                                                        66° C.                                             Reflective cooling plate 7                                                                        49° C.                                             Amount of distilled water produced:                                           Between the plates 1 and 3                                                                        1,777 ml./hour                                            Between the plates 3 and 5                                                                        1,201 ml./hour                                            Between the plates 5 and 7                                                                          913 ml./hour                                            Total               3,891 ml./hour                                            Heat efficiency     183%                                                      ______________________________________                                    

EXAMPLE 4

Distillation of sea water was carried out by employing a convection typemulti effect distillation apparatus as shown in FIG. 7, in which tendish-like vessels made of 18-8 stainless steel having a size of 2,000mm. in length, 500 mm. in width and 50 mm. in height were piled uphorizontally. Each overflow pipe attached to the bottom of the vesselprojected through the bottom to a height of 25 mm. A polyvinyl formalfoam sheet (thickness: 1.0 mm., basis weight: 138 g./m.²) was employedas the porous water-absorptive sheets stuck to the under surfaces of thebottoms of the vessels. The width of each slit opening in the verticaldirection was 2.0 mm. Sea water was supplied to the top vessel, and wasmade to overflow through the overflow pipe of each vessel in order, andwas discharged through the overflow pipe of the lowest vessel. Thetemperature of the concentrated sea water discharged from the lowestvessel was 96.8° C., and the amount of the discharged sea water was 12.0liters/hour. The heating was conducted by an electric heater. The amountof heat fed was 856 kcal./hour.

The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Temperature of sea water:                                                     In the top vessel   51.2° C.                                           In the 2nd vessel   59.3° C.                                           In the 3rd vessel   65.1° C.                                           In the 4th vessel   70.0° C.                                           In the 5th vessel   75.2° C.                                           In the 6th vessel   79.5° C.                                           In the 7th vessel   84.0° C.                                           In the 8th vessel   88.5° C.                                           In the 9th vessel   92.9° C.                                           In the lowest vessel                                                                              96.8° C.                                           Amount of distilled water produced:                                           At the top vessel   0.30 liter/hour                                           At the 2nd vessel   0.49 liter/hour                                           At the 3rd vessel   0.67 liter/hour                                           At the 4th vessel   0.83 liter/hour                                           At the 5th vessel   0.94 liter/hour                                           At the 6th vessel   1.05 liters/hour                                          At the 7th vessel   1.26 liters/hour                                          At the 8th vessel   1.52 liters/hour                                          At the 9th vessel   1.63 liters/hour                                          At the lowest vessel                                                                              1.73 liters/hour                                          Total               10.42 liters/hour                                         Heat efficiency     750%                                                      ______________________________________                                    

EXAMPLE 5

Distillation of sea water was carried out by employing a convection typesolar distillation apparatus as shown in FIG. 9, in which eightdish-like vessels were horizontally piled up. The sizes and materials ofthe vessels and the porous water-absorptive sheets were the same asthose in Example 4. The sunlight gathering box 70 was made of aluminummirror plates, and had a parabolic bottom surface and flat side walls.Also, a solar heat absorptive coating (absorption ratio: 97 to 98%)applied to the under surface of the bottom of the lowest vessel wasformed by employing a black coating composition containing an epoxyresin and carbon black. The amount of heat fed was 940 kcal./hour.

The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Temperature of sea water:                                                     In the top vessel   67.8° C.                                           In the 2nd vessel   72.0° C.                                           In the 3rd vessel   76.1° C.                                           In the 4th vessel   81.4° C.                                           In the 5th vessel   86.0° C.                                           In the 6th vessel   90.5° C.                                           In the 7th vessel   94.6° C.                                           In the lowest vessel                                                                              98.2° C.                                           Amount of distilled water produced:                                           At the top vessel   0.85 liter/hour                                           At the 2nd vessel   0.89 liter/hour                                           At the 3rd vessel   1.00 liter/hour                                           At the 4th vessel   1.28 liters/hour                                          At the 5th vessel   1.53 liters/hour                                          At the 6th vessel   1.61 liters/hour                                          At the 7th vessel   1.71 liters/hour                                          At the lowest vessel                                                                              1.86 liters/hour                                          At the heat exchanger 60                                                                          0.75 liter/hour                                           Total               11.48 liters/hour                                         Heat efficiency     684%                                                      ______________________________________                                    

What we claim is:
 1. An apparatus for distilling water whichcomprises:(a) a casing, (b) a heat receiving plate member for receivingheat on one side thereof and evaporating water to be treated on theother side thereof, (c) a plurality of cooling plate members forcondensing the water vapor and evaporating water to be treated by thelatent heat of the condensation, said cooling plate members beingarranged in parallel spaced relationship with said heat receiving platemember and with each other, each reflective cooling plate having areflective mirror surface on the condensing side, (d) water holdingmeans for holding said water to be treated on the other side of saidheat receiving plate member and on one side of each of said coolingplate members, said each water holding means being opposite to thecondensing surface of the adjacent cooling plate member, (e) a watersupply means for supplying the water to be treated to said water holdingmeans, and (f) a water collecting means for collecting the condensedwater.
 2. An apparatus for distilling water which comprisesa means forsupplying a water to be treated and a means for discharging said water,heat insulating upper and lower walls and a pair of heat insulating sidewalls, a transparent cover, both upper and lower ends of which areattached to said heat insulating upper and lower walls, a heat receivingplate and a plurality of reflective cooling plates, the heat receivingplate and the reflective cooling plates being arranged inside saidtransparent cover in that order in parallel spaced relationship with thetransparent cover and with each other, and both upper and lower endsthereof being supported by said heat insulating upper and lower walls,each reflective cooling plate having a reflective mirror surface on thecondensing side, porous water-absorptive sheets attached to the backsides of said heat receiving plate and said reflective cooling plates,upper ends of the porous sheets being connected to said means forsupplying the water and lower ends of the porous sheets being connectedto said means for discharging the water, and the porous sheet attachedto the most outer reflective cooling plate being exposed, and watercollecting means for collecting liquid water condensed on the surface ofsaid reflective cooling plates, being located at the lower portions ofthe front sides of said reflective cooling plates.
 3. The apparatus ofclaim 2, wherein said heat receiving plate has a black or dark coatingcapable of selectively absorbing the heat rays of sun on the surface. 4.The apparatus of claim 2, wherein said heat receiving plate is atransparent sheet material capable of transmitting sunlight and a blackporous water-absorptive sheet capable of absorbing the heat rays of sunis attached to the back of the transparent sheet material.
 5. Theapparatus of claim 2, further comprising a porous water-absorptive sheetfor distributing a water to be treated, to which the upper ends of saidwater-absorptive sheets attached to the back sides of said heatreceiving plate and said reflective cooling plates are connected.